The present invention relates generally to liquid crystal display (LCD) devices, and more particularly to a system, apparatus and method for improving image quality by limiting the difference between gray scale values of adjacent pixels.
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled display areas called xe2x80x9cpixelsxe2x80x9d (the smallest segment of the display). Each individual pixel is adapted to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
A LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the area of the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be a full range of colors.
The monochrome resolution of an LCD can be defined by the number of different levels of light transmission or reflection that each pixel can perform in response to a control signal. A second level is different from a first level when a user can tell the visual difference between the two. An LCD with greater monochrome resolution will look clearer to the user.
LCDs are actuated pixel-by-pixel, either one at a time or a plurality simultaneously. A voltage is applied to each pixel area by charging a capacitor formed in the pixel area. The liquid crystal responds to the charged voltage of the pixel capacitance by twisting and thereby transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions (different capacitance areas being charged of a pixel) or times depending upon the LCD illumination method. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated for only blue light to be transmitted while another for green light, and a third for red light. A greater number of different light levels available for each color results in a much greater number of possible color combinations. Colors may be combined from a red pixel, a green pixel and a blue pixel, each residing on a different LCD, to produce any desired combined pixel color. The three LCDs (red-green-blue or RGB) are optically aligned so that the resulting light from each of the corresponding RGB pixels produces one sharp color pixel for each of the pixels in the LCD pixel matrix. The LCD pixel matrix is adapted for displaying one frame of video per light strobe. Each light strobe (RGB) produces one video frame. A sequence of video frames produces video images that may change over time (e.g., motion video).
Converting a complex digital signal that represents an image or video into voltages to be applied to charge the capacitance of each pixel of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to charge the capacitance of the pixel in order to determine light transmission thereof.
Each pixel in the array of the LCD is addressed by both a column (vertical) driver and a row (horizontal) driver. The column driver turns on an analog switch that connects an analog voltage representative of the video input (control voltage necessary for the desired liquid crystal twist) to the column, and the row driver turns on a second analog switch that connects the column to the desired pixel.
The video inputs to the LCD are analog signals centered around a center reference voltage of typically from about 6.5 to 8.0 volts. A voltage equal or close to this center reference voltage is called xe2x80x9cVCOMxe2x80x9d and is supplied to the LCD Cover glass electrode which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO).
One frame of video pixels are run at voltages above the center reference voltage (positive inversion) and for the next frame the video pixels are run at voltages below the center reference voltage (negative inversion). Alternating between positive and negative inversions results in substantially a zero net DC bias at each pixel. This substantially reduces the xe2x80x9cimage stickingxe2x80x9d phenomena.
LCD technology has reduced the size of displays from full screen sizes to minidisplays less than 1.3 inches diagonal measurement, to microdisplays that require a magnification system. Microdisplays may be manufactured using semiconductor integrated circuit (IC) dynamic random access memory (DRAM) process technologies. The microdisplays consist of a silicon substrate backplane, a cover glass and an intervening liquid crystal layer. The microdisplays are arranged as a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix. To incident light, each pixel is a liquid crystal cell above a reflecting mirror. By changing the liquid crystal state, the incident light can be made to change its polarization. The silicon backplane is an array of pixels, typically 10 to 20 microns in pitch. Each pixel has a mirrored surface that occupies most of the pixel area. The mirrored surface is also an electrical conductor that forms a pixel capacitor with the ITO layer as the other plate of the pixel capacitor (common to all pixel capacitors in the matrix of pixels. As each pixel capacitor is charged to a certain pixel value, the liquid crystals between the plates of the pixel capacitors xe2x80x9ctwistxe2x80x9d or xe2x80x9cuntwistxe2x80x9d which affects the polarization of the light incident to the pixels (reflections from the pixel mirrors).
Microdisplays may have an analog video signal input (xe2x80x9canalog displayxe2x80x9d) or a digital video signal input (digital display). Analog displays, generally, are addressed in a raster mode, while the pixels in a digital display may be addressed like a DRAM, in a random order. Random access allows updating only pixels requiring updating, thus saving on processing time and associated power consumption.
A problem exists in small LCDs, especially microdisplays, which have small pixel cell areas compared to the area of the gaps between the pixel cells. Fringe fields between the pixels are therefore significant in magnitude and the area affected by fringe fields is significant with respect to the overall pixel area. This leads to image degradation of increasing severity for small LCDs and high driving voltages. Limiting the driving voltages helps, but reduces the available contrast of the LCD.
The present invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing a system, method and apparatus for improving image quality of a liquid crystal display (LCD) by modifying the video source values written to the pixels in order to smooth the magnitude of voltage transitions from one adjacent pixel to another. If the voltage transitions between adjacent pixels is too large in magnitude, the large voltage transition can generate a strong fringe field effect between the adjacent pixels.
A liquid crystal on silicon (LCoS) microdisplay is adapted to receive video information from a digital video data source. The LCoS microdisplay may operate, e.g., in a normally white twisted nematic LC mode. A rubbing direction may be selected so that disclinations appear preferably at vertical pixel borders (between columns), e.g., a 60 degree twist self-compensated reflective twisted nematic mode. If a source image with black areas surrounded by light gray areas is displayed, a white line may be observed within a gray area that borders the black area on one side thereof, while on the other side of the black area a white spot may be observed therein. If the source video image for the pixels at the border of the gray/black areas are modified, e.g., a normally black pixel written more toward gray (lighter than black but darker than the normal gray), or a gray pixel written more toward black (darker), then the resulting LCD video image has significantly less image distortion due to fringe effect fields. Such a slight reduction in the blackness of a pixel or reduction of lightness of a pixel next to a black pixel has a strong effect in the applied voltage since the electro-optical response of the liquid crystal has a small gradient close to the saturation voltage for a black pixel.
For exemplary purposes in describing the embodiments disclosed herein, a pixel voltage value (the voltage value charge on the pixel capacitor) representing black may be referred to as black or level A (00h input to an 8-bit DAC), and a pixel voltage value representing white may be referred to as white or level D (FFh input to the 8 bit DAC). Gray levels may be referred to as gray or level C (greater than blackxe2x80x9400h and less than whitexe2x80x94FFb to the 8 bit DAC).
In an exemplary embodiment of the invention, the video source data is fed into a shift register. A comparator analyzes the pixel values in the shift register and restricts all pixels values less than B that are adjacent to pixel values having at least a gray level C to substantially black video values of level B (00h less than B less than C). Alternatively, the pixels values less than level B may be reduced by a factor k (increases the gray level), if the pixel to be written to borders a gray level pixel having a gray level between C and D, where k, B and C are parameters that may be selected for the most pleasing images. For a sequential color LCD system, only one shift register need be used. For a three color (red-green-blue) LCD system, three shift registers may be used, one for each color portion of the RGB LCDs.
In another exemplary embodiment of the invention, the video source pixel data that was written to a previous row is stored in a video memory so that a comparison of previous row pixel value data can be made and the present row video source pixel data modified as describe herein. Thus both adjacent column and row pixels may be compared so that any adjacent pixel will not be written to a voltage level producing a fringe field great enough to cause image degradation (disclination).
In another exemplary embodiment of the invention, the magnitude change in adjacent pixel voltage values will be reduced by averaging the required magnitude change over a sufficient number of pixels so that no adjacent pixels will have a voltage value change larger than a desired magnitude. This may be accomplished by dividing the input video data voltage value magnitude change between adjacent pixels by the desired voltage value magnitude change to determine the xe2x80x9cnumber of pixelsxe2x80x9d over which the total video data voltage value magnitude change can be obtained without exceeding the desired voltage value magnitude change between any two adjacent pixels. This results in a more gradual change in voltage values, e.g., xe2x80x9cstair steppingxe2x80x9d adjacent pixel voltage value changes over the number of adjacent pixels until reaching the total video data voltage value magnitude change.
Adjacent pixels on the same row may be as described herein as well as adjacent pixels on adjacent rows. It is contemplated and within the scope of the present invention that a video memory may be utilized to store voltage values written to pixels on previous rows and/or columns so that no adjacent pixel has a voltage value difference great enough to cause field fringe effects.
The present invention is directed to a system for improving image quality of a liquid crystal display (LCD), said system comprising: a matrix of pixels arranged in a plurality of columns and a plurality of rows, wherein an intersection of a row and a column defines a location of a pixel in said matrix; at least one digital-to-analog converter (DAC) having a digital input and an analog output; a plurality of column switches adapted for coupling the analog output of said at least one DAC to each of said plurality of columns; a plurality of row switches adapted for selectively coupling each of said plurality of rows to said plurality of columns; column control logic for controlling said plurality of column switches; row control logic for controlling said plurality of row switches; a video frame to gray scale conversion and pixel address logic for converting video information into LCD gray scale values and corresponding pixel address locations thereof; and video data comparator/modifier logic, said video data comparator/modifier logic adapted to receive the LCD gray scale values for each pixel of the matrix of pixels, wherein gray scale values of adjacent pixels are compared and if a difference in magnitudes between the gray scale values of adjacent pixels is greater than a desired value, then at least one of the gray scale values is modified so that the difference in magnitudes therebetween is no greater than the desired value; said video data comparator/modifier logic is adapted for sending all unmodified gray scale values and any modified gray scale values to said at least one DAC; said video frame to gray scale conversion and pixel address logic adapted for sending said pixel address locations to said column control logic and said row control logic.
The present invention is also directed to a method of operation for improving image quality of a liquid crystal display (LCD) comprising a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix, said method comprising the steps of: determining if a gray scale value of a pixel is greater than a first reference value and writing the gray scale value to the pixel location, wherein if the gray scale value of the pixel is less than or equal to the first reference value, then writing a gray scale value of an adjacent pixel to the adjacent pixel, and if the gray scale value of the pixel is greater than the first reference value, then determining if the gray scale value of the adjacent pixel is less than a second reference value, if so, then writing the second reference value to the adjacent pixel, and if not, then writing the gray scale value of the adjacent pixel to the adjacent pixel.
The present invention is also directed to a method of operation for improving image quality of a liquid crystal display (LCD) comprising a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix, said method comprising the steps of: determining if a gray scale value of a pixel is greater than a first reference value and writing the gray scale value to the pixel location, wherein if the gray scale value of the pixel is less than or equal to the first reference value, then writing a gray scale value of an adjacent pixel to the adjacent pixel, and if the gray scale value of the pixel is greater than the first reference value, then determining if the gray scale value of the adjacent pixel is less than a second reference value, if so, then multiplying the gray scale value of the adjacent pixel by k and writing the product thereof to the adjacent pixel, and if not, then writing the gray scale value of the adjacent pixel to the adjacent pixel.
The present invention is also directed to a method of operation for improving image quality of a liquid crystal display (LCD) comprising a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix, said method comprising the steps of: determining if a gray scale value of a pixel is greater than a first reference value and writing the gray scale value to the pixel location, wherein if the gray scale value of the pixel is less than or equal to the first reference value, then writing a gray scale value of an adjacent pixel to the adjacent pixel, and if the gray scale value of the pixel is greater than the first reference value, then determining if the gray scale value of the adjacent pixel is less than a second reference value, if so, then dividing the difference between the gray scale values of the pixel and the adjacent pixel by I and storing the result as N and changing the next N adjacent pixel gray scale values by no more than I, and writing the changed next N gray scale values to the next N adjacent pixels, and if not, then writing the gray scale value of the adjacent pixel to the adjacent pixel.
A technical advantage of the present invention is improved image quality in microdisplays. Another technical advantage is in smoothing transitions between pixel voltages that generate strong fringe field effects. Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention.